Iron and steel machine parts having both complex geometries and accurate dimensions have been manufactured for well over a century. Traditionally such complex metallic parts were individually machined, achieving precise dimensions. However, it is faster, cheaper and easier to cast such parts using metals in the molten state, especially for mass production of parts, as is required in modern manufacturing. If casting operations are conducted properly, both uniformity of the material and efficiency of the manufacturing process can be optimally enhanced, at least in theory.
A major drawback of traditional “green sand” (a mixture of sand, clay and water) casting (like that used since antiquity), is that close tolerances in the cast part are very difficult to achieve, as is a smooth finish. The situation with “green sand” casting becomes even more problematical when mass production is involved so that variances in dimensions increase, thereby undermining interchangeability of parts. As the level of required tolerances becomes more exacting, it becomes necessary to add post-casting machining steps to the overall manufacturing process. This entails substantial expense, especially with ferrous castings.
Another problem with ferrous metal castings arises with the duty cycle to be imposed upon the finished parts. For example, grey iron is easier to produce, and has some beneficial properties, such as dampening ability. However, grey iron has intrinsically lower ductility compared to many other metals, making it useless for many applications in which a more ductile product is needed. Steel castings, while providing greater ductility, have a whole range of manufacturing difficulties, and uniformity of composition issues. The scope of such limitations is well-known in the casting art, and requires no further discussion here for an understanding of the general limitations of the conventional art of casting ferrous metals.
Up to the last sixty years, these limitations constituted serious constraints upon the usefulness of both iron and steel castings. Part of the solution was provided by the development of ductile iron, over sixty years ago. This is a well-known product that varies from standard grey iron or steel by the addition of spheroidal graphite nodules throughout the metallic matrices. The result is a high level of ductility. In contrast, in grey iron or cast iron, the carbon which is not in the pearlite portion of the product is in the form of irregular flake graphite, resulting in a relatively brittle product.
Traditionally, ductile iron has been made as carbide free as possible, for both machining and mechanical considerations. This is done in order to control the location of the carbides, which if not controlled, would form randomly or at the center of the metallic part. Such randomness is generally considered undesirable for a specifically engineered end product having close tolerances, as it can degrade or cause erratic material properties, As a result carbides are conventionally regarded as anathema to ductile iron processes.
A further description of ductile iron characteristics and methods of manufacture can be found in the Ductile Iron Handbook, the 1993 revision, American Foundrymen's Society, Inc., DesPlains, Ill.; ISBN-87433-124-2. This work is incorporated herein by reference as an example of traditional ductile iron characteristics, use, manufacturing, and limitations. Accordingly, no further description of ductile iron is necessary for an understanding of the present invention.
The use of ductile iron for casting overcomes the traditional grey iron problem of loss of ductile properties. However, the other drawbacks of the conventional art still remain. For example, the lack of reasonably close tolerances resulting from traditional “green sand” casting. More problematical is the difficulty in hardening specific portions of ductile iron castings. Conventionally this is almost impossible unless there is a secondary heat treating process. Such a process entails substantial additional expense.
The casting art became far more precise with the introduction of “lost foam” casting in 1958. The initial versions of this technique used a pattern or form made from a block of expanded polystyrene (EPS), which was supported by “green sand” during the metal pour. This process has officially been known as the full mold process. Additional developments in this technology included the use of unbonded or common sand in the process. This particular variation is now commonly known as the “lost foam” method.
The Appendix attached hereto includes an article from the American Foundrymen's Society, AFS division 11: “Lost Foam Casting”. This document is incorporated herein by reference as an example of conventional “lost foam” casting that can be applied to a variety of different metals.
Unfortunately, even with the aforementioned improvements, there are still many drawbacks in the art of manufacturing precise ferrous castings. In particular, there is still substantial difficulty in producing cast parts with appropriate (hardened) load bearing surfaces, such as those used in gears or other high-stress machinery. Even with the conventional improvements to date, modern ductile iron is not sufficiently hard for many applications, especially those that require specific, high-stress, load-bearing surfaces.
At the same time, traditional grey iron is too brittle for many extremely stressful duty cycles, such as those that would be found in many machinery arrangements, such as gears, bearing plates, and the like. Attempts to use traditional carburizing or hardening, such as that found in many steel products, leads to very complex processing that can include melting, casting, rolling, machining, heat treating, and finish machining. This is very expensive, time consuming, and extremely demanding. Such processes do not admit easily to simple and inexpensive mass production of the desired parts.
Accordingly, there is a substantial need for providing cast ductile iron or steel parts that have selected carbide surfaces to withstand high-stress duty cycles. Such a process should be inexpensive, and adapted to use existing equipment and techniques.